This invention relates to a voltage regulator for charger/generator and more particularly to a semiconductor type voltage regulator suitable for use in a charger/generator driven by an internal combustion engine.
Conventionally, in a so-called IC regulator using a power transistor to switch on and off the exciting current of the generator and particularly in the type in which the power transistor is constantly rendered on beginning with switch-on of the key switch to permit the initial excitation current to be fed from the battery to the field winding of the generator, there arises a problem that because of such an inadvertency that the key switch is left to be turned in during engine stop, the battery is discharged through the field winding within a short period of time, resulting in overdischarge.
In an approach to this problem as proposed in JP-A-54-l40112, the power transistor is so controlled as to be turned on and off at a proper duty ratio by an oscillation signal from an oscillator during an interval of time ranging from an instant that the key switch is turned in to an instant that the generator reaches a predetermined generation condition, in order to restrict the initial excitation current.
In the proposal, the oscillator has to be specifically designed for generation of the asymmetric waveform signal and in order to obtain a desired duty ratio (14.3% in the proposal), values of resistor and capacitor included in the oscillator must be adjusted. Conventionally, the voltage regulator for car on-board charger/generator is typically realized with a hybrid thick film integrated circuit and built in the generator and therefore the resistance can typically be adjusted by cutting the thick-film resistor through function trimming process, thus increasing expenditure in manufacture of IC. If a power MOS-FET is used as the switching element to allow the overall voltage regulator to be formed of an inexpensive monolithic IC, it is impossible for the resistance adjustment by trimming to be conducted.
Further, the prior art has the following disadvantage. More particularly, while the power transistor serving as switching element in the conventional IC regulator is in general connected to the low potential side of the field winding, the high potential side thereof is always applied with voltage of the battery. Accordingly, in the event of the presence of salty substance deposited on the generator rotor, leakage current flows from the slip ring adapted to pass current to the field winding on the rotor to the generator housing through the rotor shaft and bearing even when the power transistor is turned off and electrolytic corrosion tends to occur. By connecting the switching element to the high potential side of the field winding, the leakage current may be eliminated but there needs a driver power supply of a higher voltage than the battery voltage, which is difficult to realize with an IC, for driving the switching element.
In another generally known car on-board semi-conductor type voltage regulator as disclosed in, for example, British patent No. 1275986, a generator has a self-excited field winding and the field current is on/off controlled by a power transistor.
In this prior art, however, the output current of the generator is supplied to excite the generator itself and therefore when the power transistor changes from on-state to off-state or vice versa to control the exciting current, the output current changes considerably. As a result, a counter electromotive force due to that change in the output current takes place in the armature winding, causing a spike voltage which is superimposed on the output voltage and transmitted to various on-board electrical loads. Especially where the spike voltage is transmitted, directly or in the form of an electric wave, to such a communication device as an on-board radio receiver, the radio receiver is adversely affected by radio or communication noises. The noises, known as the switching noise of the generator, have hitherto been recognized as problematic in a vehicle carrying various communication devices but considered unavoidable in the switching type voltage regulator.